The present invention relates to a control device and control method for an engine of the type that switches the combustion mode.
In an ordinary vehicular engine, air sucked into a combustion chamber via an intake passage and fuel to be injected from a fuel injection valve are blended to form an air-fuel mixture. The engine generates drive power by burning the air-fuel mixture in the combustion chambers. A throttle valve is provided in the intake passage. In accordance with the amount of manipulation of the acceleration pedal by a driver (hereinafter simply called xe2x80x9cacceleration manipulation amountxe2x80x9d), the amount of air to be led into the combustion chambers is regulated by the throttle valve. Accordingly, the amount of the air-fuel mixture to be filled in the combustion chambers changes, thereby adjusting the output torque of the engine.
An electronic throttle control apparatus disclosed in Japanese Patent Laid-Open No. Hei 10-103135 executes various kinds of electronic controls associated with automatic adjustment of the engine torque, such as traction control to prevent slipping of wheels and auto cruise control to keep the vehicle speed constant. When those controls are performed, the throttle angle based on the acceleration manipulation amount is corrected according to various parameters representing the running states of a vehicle other than the acceleration manipulation amount. As a result, the amount of intake air is adjusted to adjust the engine torque to a demanded value.
Recently, engines of the type that switches the combustion mode in accordance with the running state of the engine have been proposed and made into practical use in order to satisfy both an improvement on fuel efficiency and acquisition of sufficient engine power at the same time. Such a type of engine is disclosed in, for example, Japanese Patent Laid-Open No. Hei 8-189405.
The engine disclosed in the publication is operated in homogeneous charge combustion mode in high engine speed mode or high load mode where high power is demanded. At the time of operation in the homogeneous charge combustion mode, fuel is injected into a combustion chamber in the suction stroke of the engine. The injected fuel is homogeneously mixed with air and the homogeneous mixture of air and fuel is ignited by an ignition plug.
In low engine speed mode or low load mode where very high power is not demanded, the engine is operated in stratified charge combustion mode. At the time of operation in the stratified charge combustion mode, fuel is injected into the combustion chamber in the compression stroke of the engine. The injected fuel hits against the dent that is provided at the top of the piston and is gathered around the ignition plug, thus forming an air-fuel mixture with a high fuel concentration around the ignition plug. Therefore, the ignition by the ignition plug is carried out favorably. What is more, as the average air-fuel ratio in the combustion chamber is set greater than the stoichiometric air-fuel ratio, the fuel efficiency is improved. Further, the throttle valve is opened wider than that in homogeneous charge combustion mode to set the average air-fuel ratio in the air fuel mixture greater than the stoichiometric air-fuel ratio, the pumping loss is reduced.
In the engine of the above-described combustion mode switching type, various kinds of electronic controls associated with automatic adjustment of the engine torque, such as the traction control and auto cruise control, are executed. When the engine is operated in homogeneous charge combustion mode, as mentioned above, the throttle angle is corrected in accordance with various parameters representing the vehicle""s running state other than the acceleration manipulation amount. As a result, the amount of intake air is adjusted to provide the necessary engine torque. The fuel injection amount is determined in accordance with the amount of intake air that is obtained as a result of adjusting, for example, the throttle angle, and is not determined in direct consideration of the demanded value for the engine torque.
When the engine is operated in stratified charge combustion mode, on the other hand, it is hard to change the engine torque even if the amount of intake air is adjusted by changing the throttle angle. Therefore, the engine torque is adjusted in accordance with the fuel injection amount. Specifically, the fuel injection amount is basically acquired based on the acceleration manipulation amount in stratified charge combustion mode. The obtained fuel injection amount is corrected in accordance with various parameters representing the vehicle""s running state other than the acceleration manipulation amount. As a result, the required engine torque is obtained. The throttle angle is determined according to the fuel injection amount in such a way as to become a value suitable for stratified charge combustion, and is not determined in direct consideration of the demanded value for the engine torque.
As mentioned above, the throttle angle is used as a control value for adjusting the engine torque in homogeneous charge combustion mode while the fuel injection amount is used as a control value for adjusting the engine torque in stratified charge combustion mode. If the engine torque is adjusted by different control values according to the combustion mode of the engine, however, it becomes difficult to match the engine torque characteristic between the homogeneous charge combustion mode and the stratified charge combustion mode.
To execute the homogeneous charge combustion mode favorably, the demanded engine torque value should be reflected on the throttle angle accurately. To execute the stratified charge combustion mode favorably, the demanded engine torque value should be reflected on the fuel injection amount precisely. This requires that an experiment for matching the throttle angle in homogeneous charge combustion mode with the demanded engine torque value and an experiment for matching the fuel injection amount in stratified charge combustion mode with the demanded engine torque value should be conducted beforehand. That is, an experiment should be conducted for each of two different control values of the throttle angle and fuel injection amount, so that the experiments become troublesome.
Accordingly, it is an object of the present invention to provide a device and method for engine control, which can easily match the engine torque characteristics between a plurality of different combustion modes and can simplify experiments associated with a control value for controlling the engine torque.
To achieve the above object, the present invention provides a control device for an engine that generates a torque by burning a mixture of air and fuel in a combustion chamber. The engine is operated in a combustion mode selected from a first combustion mode and a second combustion mode. The engine torque is adjusted by first adjusting means when the engine is operated in the first combustion mode. The engine torque is adjusted by second adjusting means, different from the first adjusting means, when the engine is operated in the second combustion mode. The control device includes computation means for computing a torque-reflected value reflecting an engine torque demanded when the engine is operated in the first combustion mode, based on a running state of the engine; first control means for controlling the first adjusting means based on the torque-reflected value when the engine is operated in the first combustion mode; and second control means for controlling the second adjusting means based on the torque-reflected value when the engine is operated in the second combustion mode.
According to the present invention, the torque that is demanded of an engine is reflected on one torque-reflected value even in case where either the first combustion mode or the second combustion mode is executed. When the first combustion mode is executed, the first adjusting means is controlled based on the torque-reflected value to adjust the engine torque. When the second combustion mode is executed, the second adjusting means is controlled based on the torque-reflected value to adjust the engine torque. In other words, even in case where either of the two combustion modes is executed, the engine torque is adjusted based on the torque-reflected value that is a common target control value. It is therefore possible to easily match engine torque characteristics between these different combustion modes.
The present invention provides a control method for an engine for generating a torque by burning an air-fuel mixture of air and fuel in a combustion chamber. The engine is operated in a combustion mode selected from a first combustion mode and a second combustion mode. The engine torque is adjusted in accordance with a first control value when the engine is operated in the first combustion mode. The engine torque is adjusted by a second control value, different from the first control value, when the engine is operated in the second combustion mode. The control method includes a step of computing a torque-reflected value reflecting an engine torque demanded when the engine is operated in the first combustion mode, based on a running state of the engine; a step of controlling the first control value based on the torque-reflected value when the engine is operated in the first combustion mode; and a step of controlling the second control value based on the torque-reflected value when the engine is operated in the second combustion mode.